I would like to know Why Pro players open up their finger on the non hitting arm? before our coach would just say point the ball on your non hitting arm.

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No reply?? I was curious about this one too. Actually, I am more curious about the extended fingers at the end of a serve.

I do extend my fingers during a trophy position, then I fold them when I hit. Just wondering whether the open fingers at the point of impact either at serve or forehand do anything or they are just a matter of preference.

The opening of the fingers and more importantly the extension of the wrist adds tension to the forearm which facilitates a quicker or should I say more beneficial use of the pull over of the off arm. An arm with an extended wrist "wants" to be pulled across the body and will be more connected with the torso.

Try it yourself and you will notice the difference between extended and non extended

The fingers can have any position and the opposing muscles that move the fingers can still have a large range of equal and opposing forces. That is, the muscles can be anywhere from 'relaxed' with no activation to maximum activation and force as in an isometric exercise. [Isometric means to apply forces but not to move.] It is hard to tell in the pictures whether the muscles are relaxed or supplying forces.

Demo. Look at the Nadal picture and hold your hand how he has his. While holding the same pose completely relax all muscles. Then apply maximum forces from opposing muscles without moving as is often done during isometric exercises. You can do both with the same hand position.

I believe for the serve relaxing the muscles allows for a better stretch.

I recently read this so I'm not sure of my interpretation. I used to think of the stretch shortening cycle (SSC) as the extension of the muscle-tendon assembly to near the end of its range of motion - like stretching that muscle-tendon assembly like a rubber band. Then, if the prestretched muscle-tendon is promptly allowed to shorten during the athletic movement, it adds force. Using prestretched muscles may be naturally faster also.

However, I now believe from my interpretation of some recent research that it is not the muscle-tendon assembly that stretches - it is mostly a large protein molecule called Titin in each muscle cell (sacomere). The interesting thing is that Titin can probably chose to stretch well before the muscle-tendon assembly is near the end of its range of motion - maybe at any muscle length. ??

Maybe if Nadal has applied a lot of muscle force to hold his hand as in the picture then the stretching characteristics of his muscles would be much different. For example, maybe his arm acts like a rigid mass, a counterweight? If relaxed it would not act the same. ? Federer's off hand looks relaxed. ?